EP0634389B1 - Process for the preparation of dialkyle carbonates - Google Patents

Description

The present invention relates to a process for the preparation of dialkyl carbonates by reacting carbon monoxide (CO) with alkyl nitrites in the presence of a heterogeneous platinum metal catalyst, the activity of the catalyst being maintained by adding small amounts of halogen.

Dialkyl carbonates are of general chemical and technical importance. For example, diethyl carbonate is an excellent medium-boiling solvent. The dialkyl carbonates are also excellent carbonylation and acylation reagents. They are of great importance in the production of other carbonates, urethanes and ureas. Finally, due to their high oxygen content, they are suitable as fuel additives to improve the knock resistance of petrol.

It is known to produce dialkyl carbonates by reacting phosgene or alkyl chloroformates with alcohols.

However, there is an increasing interest in replacing the use of the toxic phosgene or the intermediates derived therefrom, such as chloroformate, by other processes.

Processes in which carbon monoxide is reacted in the gas phase with alkyl nitrite on a platinum metal catalyst are particularly important here. This process is based on reaction equation (1) (example dimethyl carbonate).

For this purpose, methyl nitrite itself can be produced in a manner known per se in an upstream reaction in accordance with one of the reaction equations (2) - (5) .

• JP 60/181 051; X.-Z. Jiang et al., Cuihua Xuebao 10(1), 75-78 (März 1989);
• EP 425 197; X.-Z. Jiang, Platinum Metals Rev. 34(4), 178-180 (1990); EP 464 460; EP 503 091; EP 501 507; EP 503 618; EP 523 508; EP 523 728; EP 538 676.

The production of dimethyl carbonate by reacting carbon monoxide and methyl nitrite in the gas phase in the presence of a heterogeneous catalyst, which is preferably a supported catalyst containing platinum metal, particularly preferably a supported catalyst containing palladium and particularly preferably a supported catalyst containing palladium halide, has been described in various ways , for example in the following scientific publications or patent publications:

• JP 60/181 051; X.-Z. Jiang et al., Cuihua Xuebao 10 (1) , 75-78 (March 1989);
• EP 425 197; X.-Z. Jiang, Platinum Metals Rev. 34 (4) , 178-180 (1990); EP 464 460; EP 503 091; EP 501 507; EP 503 618; EP 523 508; EP 523 728; EP 538 676.

A technical embodiment of this process is described in patent application EP 523 728. This patent application includes the recycling of the nitrogen oxides released in the course of the reaction of methyl nitrite with carbon monoxide, together with the unreacted gaseous reactants and the additional gas required for inerting, preferably nitrogen, in a process step upstream of the actual production process of the dimethyl carbonate, which corresponds to reaction equation (2) and in the methyl nitrite required for the reaction by feeding in methanol and oxygen and with the greatest possible removal of the water liberated in the process is regenerated. Thus, based on the gaseous components involved, specifically with regard to the inert gases and auxiliaries, the unreacted gaseous reactants, such as, for example, the unreacted methyl nitrite and carbon monoxide, and the nitrogen oxides involved, it is a cyclic process.

The actual production process for, for example, dimethyl carbonate, which is described by reaction equation (1) , takes place on a heterogeneous catalyst which is located inside a tube bundle reactor. A disadvantage here is the high temperature load on the catalyst which results from the high heat of reaction of the reaction between carbon monoxide and methyl nitrite which proceeds according to reaction equation (1) .

The thermally labile methyl nitrite is easily decomposed under these conditions.

It is therefore unsurprising that 0.5% by weight of formaldehyde dimethyl acetal, based on dimethyl carbonate formed, is found in the crude product in the procedure described in said patent application EP 523 728. However, such impurities cannot be accepted for some uses of dimethyl carbonate, which necessitates extensive separation and purification steps.

If, for example, the formaldehyde dimethyl acetal mentioned is a low-boiling substance, there are further problems due to the inevitable accumulation of such volatile components within the entire underlying cycle process Portions of the circulating gas and the condensed reaction products, if it is not dimethyl carbonate itself, are discharged continuously or discontinuously, preferably continuously. This causes raw material losses and complex exhaust gas treatment is required.

It was therefore the task of carrying out the reaction of the gaseous reactants carbon monoxide and alkyl nitrite over a heterogeneous catalyst in such a way that that the heat of reaction liberated is rapidly dissipated and the occurrence of decomposition products of methyl nitrite is minimized.

Continuous thermostatting must be set up for reactions with extremely strong heat and catalysts or reactions with extremely sensitive temperature behavior.

Here, the catalyst can be accommodated, for example, either between the tubes or also within the tubes of a heat exchanger. (Linde reactor according to patent application DE-OS 34 14 714, or tube bundle reactor according to chemical engineering technology 51 (1979), pp. 257-265).

Reactors of this type with tube diameters from one to several cm and tube lengths of 2-20 m have long been state of the art.

Despite the constant heat flow in the radial direction, especially in larger reactors, for example in the synthesis of dimethyl carbonate from methyl nitrite, a "hot spot" can form inside the reactor tubes, which is responsible for loss of selectivity due to the undesirable formation of formaldehyde dimethyl acetal.

Surprisingly, it was found that the synthesis of dialkyl carbonate by continuous reaction of carbon monoxide with alkyl nitrite over a heterogeneous catalyst containing one or more platinum metals, preferably containing palladium, can be achieved in the gas phase with continuous temperature control without the formation of by-products in that the catalyst bed in the form one or more regularly shaped flat layers with a thickness of 0.01-50 cm is formed, the surface of the catalyst layers being closed off by a layer permeable to gases, and this surface being spaced apart on the educt inflow side and / or on the product outlet side 0.1 - 10 cm of an equally regularly shaped wall, which separates the space for the substances to be reacted from the space for the heat transfer medium, and such guidance of the substances to be reacted is carried out that this catalyzes Flow through the ator bed perpendicular to the flat catalyst layers.

The accompanying drawings show the basic structure and the principle of operation of the reactors according to the invention and some possible technical variants.

In the accompanying drawings, the arrows with solid lines and the reference number (1) denote the respective material flow from the feed of the educt (1a) (starting material / substance mixture, optionally together with a diluent or carrier gas) for removing the product (1b) ( Reaction product / reaction product mixture, optionally with diluent / carrier gas). The arrows with dashed lines and the reference number (2) mean the heat flow from the catalytic converter to the heat transfer medium.

The dotted, narrow surfaces (3) mean that the catalyst bed is closed off by a gas-permeable layer; the layer permeable to gases is not particularly emphasized in the drawings. The horizontally dashed areas (4) mean the space closed off by a wall for the heat transfer medium.

In a manner known to the person skilled in the art, the heat transfer medium is considered to be moved by the reactor according to the invention and to be connected to an external cooling for temperature control. Where it is to be shown by way of example in the drawings, the heat transfer stream is denoted by arrows with a double line and the numbers (4a) for the entry and (4b) for the exit.

Figure 1 shows the principle of the reactor according to the invention on a section through a flat, flat catalyst bed with opposite heat transfer spaces. FIG. 2 shows the principle on the section through a catalyst bed designed as a hollow cylinder. In contrast, FIG. 3 shows a tubular reactor according to the prior art, likewise in section. While the material and heat flow in the reactor according to the invention always run parallel, antiparallel or both parallel and antiparallel to one another, the material and heat flow are orthogonal to one another in the conventional reactor. Consequently, in the reactor according to the invention the surfaces through which the material and heat flow flow are always identical, whereas they cannot be in the conventional reactor.

Figures 4 - 9 show variations of Figure 1 and Figure 2 , in which in a modification of the construction - flat catalyst bed or hollow cylinder as a catalyst bed or supply and removal of educt and product - a parallel or anti-parallel or both parallel and anti-parallel material and Heat flow guidance in the catalyst bed is shown. The heat flow is to be understood here as the one that flows in the direction from the catalyst bed to the wall of the heat transfer chamber. The additional numerical designation of the drawing symbols of the Übersichlichtkeit was half in Figures 4 - omitted. 9 It can be seen that FIGS. 4 and 5 are assigned to FIG. 1 and FIGS. 6-9 to FIG . 2 . In FIGS. 4 and 5 , the heat flow moves both parallel and antiparallel to the material flow, in FIGS. 6-9 this occurs either parallel or antiparallel.

The catalyst bed can be formed in one or more flat layers. Thus, the flat catalyst layers and the opposite, regularly shaped walls of the heat transfer spaces can be combined in a package-like arrangement (see FIG . 10 ). The combination of flat catalyst and heat transfer chamber can, however, also be arranged in a "rolled up" cylindrical shape (see FIG . 11a ). If here in the overall arrangement the material and heat transfer flows are also orthogonal to one another, this does not apply in detail to the catalyst bed in the manner described above, as is shown in FIG. 11b .

Figures 12-15 show possible technical embodiments in which tube inserts are used in the manner of conventional tube reactors. FIGS . 16-19 show individual tube inserts for such reactors, which can be screwed in, welded in or used in the reactor body in another, generally known manner.

The reactors that can be used according to the invention are characterized in that the inflowing starting material strikes the largest possible catalyst surface and at the same time the largest possible surface for heat exchange is realized. This means that for a given space-time load there is a very low inflow load and the temperature of the catalyst is uniform. The in turn means that in the reactors according to the invention, unlike in the known reactors with continuous thermostatting, the heat flow with respect to the material flow through the catalyst is conducted in parallel, antiparallel or in parallel and antiparallel. In connection with this, the catalytically active beds in the reactors according to the invention have a particularly small expansion in the direction of flow. This ranges from 0.01 to 50 cm, preferably from 0.02 to 20 cm, particularly preferably from 0.05 to 10 cm, very particularly preferably from 0.2 to 2 cm. The catalyst distribution in the reactors according to the invention is limited by porous walls which, in addition to fixing the catalyst bed, have the task of guaranteeing a uniform flow through the catalyst bed over the entire surface. For this purpose, the flow resistance of these walls and the catalyst bed must exceed a certain minimum value, which can easily be determined experimentally or by calculation by a person skilled in the art. In general, the minimum flow resistance of the catalyst beds including the porous gas distribution walls should have values from 1 mbar to 10 bar, preferably from 2 mbar to 1 bar, particularly preferably from 5 mbar to 500 mbar.

The distance between the catalyst bed and the wall is 0.1 to 10 cm, preferably 0.2 to 8 cm, particularly preferably 0.3 to 6 cm, very particularly preferably 0.4 to 4 cm.

The porous walls in the reactors according to the invention preferably consist of sintered materials with a corresponding porosity, as are known to the person skilled in the art for gas distribution. Sintered moldings made of metals are particularly preferred because of their good thermal conductivity. Pinhole constructions can also be used. They too must meet the requirements to distribute the feed gas stream over the catalyst without major local fluctuations and to ensure a uniform flow through the bed; this also requires a sufficient flow resistance, which can also be generated at least partially by the catalyst bed. For this purpose, the catalyst material must be sufficiently fine-grained to ensure the required pressure drop at a sufficient level at the low penetration depth. It is also understandable that the catalyst must have a sufficient flow to be able to in the process of Ensure a uniform pack when filling; for example, the catalyst material must not tend to stick. The relationship between particle shape and particle size on the one hand and the flow resistance of a bed of such particles on the other hand is known to the person skilled in the art and can be determined experimentally.

The regularly shaped flat layers of the catalyst bed are preferably hollow cylinders which are closed on one side and have diameters of 2 to 200 cm, in the case of apparatus on an industrial scale preferably 7 to 100 cm, particularly preferably 10 to 50 cm; they are also preferred in the form of larger aggregates, e.g. arranged as a tube bundle.

Alkyl in starting materials and products has 1 to 4 carbon atoms and is, for example, methyl, ethyl, propyl, i-propyl, butyl or i-butyl, preferably methyl or ethyl, particularly preferably methyl.

Platinum metal catalysts contain one or more metals of the platinum metal group such as Pd, Ru, Rh, Pt or Ir, preferably Pd or Pd in a mixture with another platinum metal, particularly preferably Pd alone. The platinum metal is present as a salt, preferably as a halide; it can also exist as a complex, for example as Li ₂ PdCl ₄ . Carriers are, for example, coal, Al ₂ O ₃ (for example 103 \ "symbol" -Al ₂ O ₃ ), SiO ₂ , aluminosilicates, zeolites and others familiar to the person skilled in the art.

The process according to the invention is carried out at a pressure of 0.1 to 10 bar and a temperature on the catalyst of 50 to 150 ° C; the temperature is maintained or controlled continuously by a heat transfer medium.

The molar ratio is alkyl nitrite: CO = 0.1-10: 1, preferably 0.2-4: 1, particularly preferably 0.3-3: 1. In a known manner, the reaction partners are treated with an inert gas and / or the underlying C ₁ -C ₄ alkanol added.

example a) Catalyst production

10 ml of γ-aluminum oxide pellets with a diameter of 2-3 mm were soaked in a solution of lithium tetrachloropalladate in water and dried at 60-80 ° C in a vacuum. The catalyst then contained 8 g / l palladium.

b) Description of the process

The reactor consisted of an oil-thermostatic glass tube (length 62.0 cm, inner diameter 4.9 cm). A tube made of glass sintered material was located concentrically inside the thermostatted glass tube. This tube (length 15.0 cm, diameter 2.0 cm, porosity 3) was sealed at the upper end and melted at the lower end to an evacuated double-walled glass tube with a conical glass section (normal grinding NS29) (inner glass tube with a diameter of 2.4 cm, outer Glass tube with a diameter of 3.8 cm); the space between the inner and outer glass tube was evacuated. The product gas was discharged through the inner glass tube. The outer glass tube had a length of 25.0 cm, measured from the NS29 conical section to the lower end of the thermostatted glass tube. At the end of the thermostatted glass tube, the outer glass tube was sealed gas-tight with the thermostatted glass tube. Concentric to the inner glass sintered tube was another glass sintered tube inside the oil-thermostatted glass tube (length 15.0 cm, inner diameter 3.2 cm, wall thickness 2 mm, porosity 3). At the lower end, this glass sintered tube was melted onto an NS29 ground glass sleeve, which was seated gas-tight on the core of the vacuum double jacket tube. The outer glass sintered tube was fused at the top with a 10.0 cm long glass tube with NS29 sleeve. An approximately 10.0 cm long, evacuated ground glass plug was seated gas-tight in this sleeve.

The catalyst bed was located between the outer and inner glass sintered tube.

The oil jacket of the reactor was thermostatted to 90 ° C and a gas mixture of 55 vol .-% nitrogen; 20 vol .-% methyl nitrite, 20 vol .-% carbon monoxide and 5 vol .-% methanol passed over the catalyst. 200 ppm HCl (volume) was added to the gas mixture. The gas flowing out of the reactor was cooled to 5 ° C, and the condensed phase obtained was examined by gas chromatography.

The uncondensed products were detected by means of IR spectroscopy and mass spectroscopy.

In the reaction product containing the dimethyl carbonate formed, neither dimethyl oxalate nor methyl formate and formaldehyde dimethyl acetal could be detected analytically.

c) Comparative example

In a vertical tubular reactor (glass, length 50 cm, diameter 4 cm) 10 ml of the catalyst from example a) were introduced between a filling of Raschig rings.

The glass tube was heated to 90 ° C. and a gas mixture of 55% by volume nitrogen, 20% by volume methyl nitrite, 20% by volume carbon monoxide and 5% by volume methanol was passed through, with this gas mixture containing 200 ppm hydrogen chloride gas ( Volume) were added. The gas flowing out of the reactor was cooled to 5 ° C, and the condensed phase obtained was examined by gas chromatography.

The uncondensed products were detected by means of IR spectroscopy and mass spectroscopy.

0.03% by weight of dimethyl oxalate, 0.1% by weight of methyl formate and 0.04% by weight of formaldehyde dimethyl acetal were detected in the dimethyl carbonate obtained.

Claims (10)

1. Process for the preparation of dialkyl carbonate by continuous reaction of carbon monoxide with alkyl nitrite in the gas phase on a heterogeneous catalyst, preferably on a heterogeneous catalyst containing one or more platinum metals and particularly preferably on a heterogeneous catalyst containing palladium, with continuous temperature control using a heat transfer medium, characterized in that the catalyst bed takes the form of one or more flat layers of regular shape with a thickness of 0.01 - 50 cm, the surface of the catalyst layers being sealed by a gas-permeable layer, and this surface, on the starting-material feed side and/or on the product discharge side, facing a wall of similar regular shape, at a distance of 0.1 - 10 cm, which separates the space for the substances to be reacted from the space for the heat transfer medium, and the substances to be reacted being introduced in such a way as to flow through the catalyst bed perpendicularly to the flat catalyst layers.
2. Process according to Claim 1, characterized in that the catalyst bed has a thickness of 0.02 - 20 cm, preferably 0.05 - 10 cm and particularly preferably 0.2 - 2.0 cm.
3. Process according to Claim 1, characterized in that the distance between the catalyst bed and the wall is 0.2 - 8.0 cm, preferably 0.3 - 6.0 cm and particularly preferably 0.4 - 4.0 cm.
4. Process according to Claim 1, characterized in that the gas-permeable layer for sealing the catalyst layers takes the form of meshwork, a perforated screen or sintered material, preferably sintered material and particularly preferably sintered metal.
5. Process according to Claim 1, characterized in that the flat catalyst bed, including its gas-permeable sealing layer, has a minimum flow resistance of 1 mbar to 10 bar, preferably of 2 mbar to 1 bar and particularly preferably of 5 - 500 mbar.
6. Process according to Claim 1, characterized in that the regularly shaped, flat layers of the catalyst bed take the form of hollow cylinders, sealed on one side, with diameters of 2 - 200 cm, preferably 7 - 100 cm and particularly preferably 10 - 50 cm, several of them being arranged in the form of relatively large aggregates, particularly preferably as tube bundles, in a preferred embodiment.
7. Process according to Claim 1, characterized in that it is carried out at a pressure of 0.1 - 10 bar.
8. Process according to Claim 1, characterized in that the catalyst temperature is 50 - 150°C.
9. Process according to Claim 1, characterized in that it is carried out with an alkyl nitrite:carbon monoxide volume ratio of 0.1 to 10:1, preferably 0.2 to 4:1 and particularly preferably 0.3 to 3:1.
10. Process according to Claim 1, characterized in that a palladium salt applied to a support is used as the catalyst.

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